The present invention is directed to an optical comb generator for generating a comb spectrum of spectral lines lying at equidistant optical frequencies from a spectral line generated by a coherent reference light source having a defined coherent time and lying at a defined optical reference frequency, said generator including an annularly closed optical waveguide, means for coupling the optical reference frequency output from a reference light source into the waveguide for circulating in the waveguide with a defined round-trip time and for coupling light from said waveguide, and a single-sideband modulator means arranged in the waveguide to offset the light circulating in the waveguide by a defined, optical frequency shift at every round-trip therein.
For stabilizing the transmission laser of a coherent optical multi-channel transmission system, an optical comb generator is required that produces a set of equidistant reference frequencies from the light of a reference laser so that the respective transmission laser can be bound to a respective reference frequency.
One possible realization of such a comb generator is described in an article in Electronics Letters, Vol. 26, No. 1, Jan. 4, 1990, pp. 28-30. In this comb generator, the light of the reference laser is supplied via a fiber coupler into an annularly closed light waveguide. A single-sideband modulator will offset the frequency of the light by a defined frequency shift. One part of the light is coupled out of the waveguide through an optical coupler and the remainder of the light traverses the ring again and is, again, frequency-offset, etc. When the round-trip losses are compensated by an optical amplifier, then the width of the comb spectrum that can be achieved is only dependent on the frequency response of the single-sideband modulator, of the optical amplifier and of the fiber coupler. The optical isolator only prevents the propagation of the light in the waveguide in an undesired round-trip direction.
Single-sideband modulators having a high harmonic distortion attenuation of, for example, more than 30 dB have heretofore only existed only as acousto-optical modulators, for example in the form of Bragg cells wherein, however, the maximum operating frequency amounts to only a few GHz. Over and above this, the conversion efficiency, which is defined by the relationship between the amplitude of the signal frequency-shifted by the specific frequency shift and the amplitude of the light preceding the single-sideband modulator, decreases given high frequencies and only amounts to a few percent for frequencies of more than 1 GHz.
Electro-optical single-sideband modulators effortlessly reach operating frequencies of more than 10 GHz. However, a harmonic distortion attenuation of more than 30 dB given a simultaneously high conversion of efficiency of more than 0.9 can only be achieved for extremely small fabrication spreads.
When the single-sideband modulator generates a plurality of modulation sidebands, then a superposition of the sub-waves of the same frequency from different round-trip cycles occurs in the annularly closed waveguide. Dependent on the excitation of the undesired modulation sidebands, the phase shift between the individual sub-waves, the modulation frequency of the single-sideband modulator that defines the frequency shift, as well as the round-trip time required for a round-trip in the waveguide, the power in the individual spectral lines differs greatly, individual spectral lines can disappear or the annularly closed waveguide can even resonate.
The problem can be overcome if the reference laser is pulsed (see, for example, EFOC/LAN Conference Volume, 1990, pp. 314-319). Here, the pulse duration should be selected no greater than the round-trip time, and the pulse spacing should be selected of such a size that no interference from light of the neighboring pulses occurs in the ring. The average power in each spectral line is reduced, however, according to the pulse-duty factor so that the signal-to-noise ratio required for reliable frequency control can no longer be achieved under certain circumstances.